Telephones are often operated using keys, each of which provides a particular function. In some situations, it is helpful to have dozens of keys. For example, a receptionist who supports several people may want to have one key per person that can be used to transfer calls to that person and/or indicate whether that person's phone is in use. Rather than incorporate all of these keys into a phone, phone manufacturers have created a separate device, called a “key expansion module,” that includes additional keys that can be used in conjunction with a phone.
In order to operate, a key expansion module (KEM) has various requirements. For example, a KEM needs to communicate with its associated phone in order to transmit key presses. A KEM also needs a source of power. And, if a KEM's keys are programmable, it needs a way to configure itself. One solution is to attach the KEM to its associated phone. Once the KEM and the phone are attached, they can communicate, and the phone can provide the necessary power and configuration.
Unfortunately, attaching the KEM to the phone means that the phone must be specially-designed. Since the phone needs extra hardware to support the KEM, the phone will be more expensive. If the phone is not used with a KEM, this extra hardware will be superfluous and will impose an unnecessary cost on the customer.
Also, the interface between the phone and the KEM (both physical and communicative) is often fixed. For example, the phone and the KEM might snap together to form a single device. Also, they might communicate using an external serial bus with a proprietary protocol. In these situations, a particular KEM is used with a particular phone. If the phone changes and/or the KEM changes, they may no longer be compatible.
What is needed is a key expansion module that can be used in conjunction with any phone.